Insecticides can be labeled as a per acre rate, or a per 100 gallons rate. Growers like the 100 gallon rates, because they are easy to mix, but often the per acre rates are more accurate.
When spraying with a mistblower, it's pretty easy to calibrate the sprayer. In fact, most growers can tell you exactly how much water they are putting out per acre. The problem comes when putting out sprays with a high pressure sprayer. Yet, when I start to question people, they can usually give me a ball park figure -- say less than 200 gallons per acre or around 300 gallons per acre or 500+ gallons per acre. And that's all you need to know to get an idea of how much chemical to add per 100 gallons.
For instance, if you are trying to apply Envidor at 24 ounces per acre, it you are using 300 gallons of water per acre, mixing the product at 8 ounces per 100 gallons will give you the exact rate. But if you are spraying 500 gallons of water per acre, you will be applying more than 40 ounces per acre which exceeds the rate and will cost way too much money. If you're using less than 200 gallons of water per acre, you'd best mix it at 12 ounces per 100 gallons. So before you mix, think about how much water you're likely to use. *(See below for calibrating a high pressure sprayer).
Using that same Envidor at 24 ounces per acre out of a mistblower? Applying 40 gallons of water per acre? Then you would mix the Envidor at 60 ounces in 100 gallons.
But what about when you are applying a materials with a mistblower that has a per 100 gallon rate? Asana is a good example of this. Labeled at 9.6 ounces per 100 gallons, it can be used in a mistblower to get control of multiple pests including twig aphids, balsam woolly adelgid, and elongate hemlock scale.
Back when I first started working, Bill Huxster and Jim McGraw encouraged people to use 3 times the per 100 gallon rate when spraying with a mistblower. That's because growers are using much less water, but they still need basically the same amount of chemical per acre. It turns out, that's a pretty good rule of thumb. A mix of 30 ounces of Asana per 100 gallons seems like a lot, but Asana is labeled at up to 52 ounces per 100 gallons in ultra-low volume sprays (such as an aircraft) and as much as 37 ounces per acre for certain pests in Christmas trees.
When looking at labels, see if there is a cap for application. Is there an amount you should not exceed when applying materials specified on the label? Use this as your guideline for how much to apply.
Figuring out rates when it comes to insecticides is just common sense. Take a minute to make sure what you're using makes sense.
*CALIBRATING A HIGH PRESSURE SPRAYER:You can always calibrate your high pressure sprayer. First determine how much water you are putting out per second. Spray into a bucket for 10 seconds having set up the gun the way you mean to spray, measure what you spray and divide by 10. (You might need to cover the bucket with a garbage bag to keep it from coming out). That gives you what you are spraying out per second. Then spray a few trees and see approximately how many seconds it takes to cover the tree the way you want to.
An example -- say you spray 3 ounces per second and approximately 4 seconds per tree on one side. Since you spray from two directions, thats 8 seconds per tree. That's 24 ounces of water per tree. If you have 1,700 trees per acre, you are using about 300 gallons per acre. If you stock at 2,200 trees per acre, you would be using about 400 gallons per acre.